Typically, in a motor used in a compressor of an air conditioner, a motor protector is provided for sensing abnormal heat generation or extraordinary electric current that would be produced in the case where the rotor of the motor has been restrained or where an excessive load has been added to the rotor.
Such a motor protector 101 as is known in the art is shown in FIG. 18. This motor protector 101 has an adjusting screw 103 centrally located in an open ended cylindrically shaped base 102 typically made of an electrically insulating material such as resin. A disk-shaped snap acting bimetal disk shaped element 104 is installed at one end of adjusting screw 103 internal to base 102 with movable contacts 105 and 106 mounted at both edges of the bimetal disk 104.
A pair of fixed contacts 107 and 108 are positioned near an inner wall of base 102 so as to be able to contact the movable contacts 105 and 106. One of the fixed contacts 107 is connected to a heater 109 which in turn is connected to connecting terminal 110. The protector is positioned so that in an electrical circuit providing electric current to a motor (not shown), the current will flow from the fixed contact 108 to a connecting terminal 110 through the bimetal disk 104, fixed contact 107 and heater 109.
When the rotor of the motor has been restrained by some reason or when an excessive load has been added to the rotor of the motor, the ambient temperature surrounding protector 101 rises and/or an excessive current (which is greater than normal) flows through the bimetal disk 104 which results in the bimetal disk 104 increasing in temperature and snapping over as is shown (dotted lines) in FIG. 18. In such conditions, the movable contacts 105 and 106 and the fixed contacts 107 and 108 separate and the power supplied to the motor is cut off thereby protecting the motor.
Such a protector has proven very useful in operation, however, there are certain instances when an improved motor protector device would be desirable.
For example, in conventional motor protector 101, only one bimetal element (one switch) is used to effect protection, both at the time when the rotor has been restrained or at the time of an excessive load, thereby requiring the temperature for the snap-action of the bimetal 104 to be set at a high temperature level (in the range between 140 and 160 degrees centigrade).
Also, in the case of the prior art motor protector used with the motor of a compressor of an air conditioner, etc., the elevation of the temperature of the motor stemming from a leakage of the cooling gas is not detected thereby possibly causing problems with protection of the motor.
Still further, in motor protector 101 of the prior art, if the fixed contacts 107 and 108 and the moving contacts 105 and 106 are welded together by electrical arcing and melting of the contact material, the electric current will not be cut off to the motor even in the case where the rotor is restrained thereby creating a potentially dangerous situation. Also, there is a possibility of bimetal disk 104 no longer performing its function as it comes to the end of its mechanical life which could create a dangerous situation.